Now we have an official statement from Gianluca Martino, a member of the five-headed Arduino team.
I had written this request to the Arduino Blog:
[ch8222]Since the Arduino Duemilanove is equipped with a (FDN)340 instead of the original MOSFET NDT2955 the switching-off of the 5V isn[ch8217]t still working. See my posts:http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1284034373
I have noticed it at 5 boards, labeled: Made in Italy, designed by tinker.it[ch8220]
Gianluca Martino replied:
[ch8222]The power Switch is intended for switch the supply from one power source to another and not to remove the 5v from the USB port. This is necessary to keep the ftdi live for an insertion of the USB.[ch8220]
Let me sum up:
The fact is, the precursors of the Duemilanove, the Diecimila Arduino and the NG Arduino were equipped with Power Jumpers:http://www.ladyada.net/learn/arduino/lesson0.html
And it was necessary to set it in the right position by hand before starting.
The new Duemilanove has an automatism which should do this hand job for the user by switching a MOSFET. And the producer of the boards wouldn't had introduced it if it wouldn't be necessary.
And as long as they have used the NDT2955 all was working perfect. But now with the FDN340 it doesn't work! And we know the reason why. Then it would have been cheaper to substitute the MOSFET by a simple wire bridge.
It would be interesting to know the opinion of the hardware specialists of this forum regarding the statement of Gianluca Martino. And of course of mine.